Activation of the mitochondrial permeability transition pore (PTP) clearly plays a key role some of the most wide-spread and therapeutically challenging human diseases. Our studies have established that the PTP operates in two modes 1) transiently, whereby the PTP acts as a mitochondrial Ca2+ release channel or 2) persistently, which ultimately results in the cell death and disease. Although well characterized on a functional level, we have no small molecules that specifically target the PTP or the transition from transient to persistent - from normal to pathological. As a result, our goal in this applicatin is to use the resource available in the MLPCN network is to identify the probes that uniquely target the PTP. This information is critical if we are to be able to effectively identify and/or design valuable therapeutics targeting the transition of the PTP from normal to pathological. The specific objectives of this application are based in the synergy possible through the unique combination of novel approaches available in our three laboratories; Aim 1 - We will screen the available NIH SMR to identify small molecule probes able to inhibit PTP opening using a simple in vitro assay that has already been adapted to the 1536 plate format to allow screening in high throughput (HTS) formats. Aim 2 - Since the primary screen is designed to caste a wide net, secondary screens have been developed that can also be used in HTS formats to limit to our future studies to molecules that specifically target the PTP. Aim 3 - We will initiate studies on te mechanism of action of active compounds based on assays of mitochondrial function as assessed in an in situ, whole cell context. These tertiary screens will also serve as a mechanism to assess chemically modified active compounds in an attempt to improve their biological activity. These studies will set the stage for future interrogation aimed at extending our understanding of mitochondria and PTP activity in physiological and pathological settings. Clearly, these outcomes will be fundamental to developing novel therapeutic strategies specifically targeting the pore in the many disease processes in which the PTP has been clearly implicated.